Hydrostatic transmission

ABSTRACT

A driven main pump drives a hydraulic motor through high pressure and low pressure conduits, while a feed pump supplies pressure fluid through a check valve to the low pressure conduit. A control conduit connects the feed pump with a pressure maintaining valve opening into a low pressure container, and includes a shiftable control valve operated by fluid from the high pressure conduit, but connecting in all positions the feed pump with the pressure maintaining valve. In the control conduit portion between the control valve and the feed pump, a throttle is provided whose pressure differential is greater than the pressure differential of the check valve between the feed pump and the low pressure conduit.

United States Patent [191 Tittmann et al.

HYDROSTATIC TRANSMISSION Inventors: Egon Tittmann, Leonberg; WernerRomeisch, Gerlingen, both of Germany Robert Bosch GmbI-I, Stuttgart,Germany Filed: Mar. 15, 1973 Appl. No.: 341,609

Assignee:

Foreign Application Priority Data Mar. 17, 1972 Germany 2218472References Cited UNITED STATES PATENTS 9/1941 Kendrick 60/329 3/1965Firth et al. 60/DIG. 5

[ July 16, 1974 3,230,699 l/l966 l'lann et al. 60/448 X 3,360,934 1/1968Moyer et a1. 60/329 3,429,123 2/1969 Burroughs 60/451 X 3,650,108 3/1972lsaac 60/451 X Primary ExaminerEdgar W. Geoghegan Attorney, Agent, orFirm-Michael S. Striker [57] ABSTRACT Adriven main pump drives ahydraulic motor through high pressure and low pressure conduits, while afeed pump supplies pressure fluid through a check valve to the lowpressure conduit. A control conduit connects the feed pump'with apressure maintaining valve opening into a low pressure container, andincludes a shiftable control valve operated by fluid from the highpressure conduit, but connecting in all positions the feed pump with thepressure maintaining valve. In the control conduit portion between thecontrol valve and the feed pump, a throttle is provided whose pressuredifferential is greater than the pressure differential of the checkvalve between the feed pump and the low pressure conduit.

10 Claims, 3 Drawing Figures HYDROSTATIC TRANSMISSION BACKGROUND OF THEINVENTION The German published application 2,223,120 discloses ahydrostatic transmission comprising a hydraulic main pump, a hydraulicmotor connected by high pressure and low pressure conduits with thehydraulic main pump, and a feed pump which pumps pressure fluid throughcheck valves into the respective low pressure conduit, and which is alsoconnected with a flush ing control valve through which heated pressurefluid from the low pressure conduit is discharged by a pressuremaintaining valve downstream of the flushing control valve.

The transmission of the prior art has the disadvantage that for theflushing control valve and also for the feed pump, separate pressuremaintaining and feeding pressure safety valves are required, so that thetransmission is complicated.

SUMMARY OF THE INVENTION It is the object of the invention to provide ahydrostatic transmission of the above-described type, which is of simpleconstruction, and can be manufactured at a lower cost than the apparatusof the prior art.

With this object in view, the present invention provides a throttlebetween the feed pump and the control valve, while the control valve isconstructed in such a manner that in all positions of the control valve,the

feed pump communicates with the pressure maintaining valve, and throughthe same with a low pressure discharge area.

Due to this novel arrangement of the invention, no pressure limitingvalve is required for the feed pump. The throttle between the feed pumpand the flushing control valve, assures the function of the flushingcircult.

A further advantage of the invention resides in that the throttleobtains a speed-depending increase of the feeding pressure during idlingof the hydrostatic transmission, permitting an adjustment of the mainpump in this condition even at high rotary speeds.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram schematicallyillustrating a hydrostatic transmission according to an embodiment ofthe invention;

FIG. 2 is a sectional view illustrating a first control valveconstruction which may be used in the embodiment of FIG. 1; and

FIG. 3 is a sectional view illustrating a second control valveconstruction which may be used in the embodiment of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An adjustable and reversiblehydraulic main pump is driven by prime mover 11, which also drives afeed pump 12 in synchronism with the main pump. The main pump 10 isconnected by pressure conduits 13 and 14 with a hydraulic motor 15,which may drive the wheels of a vehicle.

The feed pump 12 pumps through an outlet conduit 12a, in which a filter24 is provided, into a control conduit 16 which is connected with apressure maintaining valve 18 biassed by spring 29 and being subjectedto the pressure of the control conduit by a connecting conduit 28 sothat the pressure maintaining valve 18 discharges at a certain pressureinto the open container 19. Two outlet conduits 20 and 21 branch off theoutlet conduit 12a of the feed pump and are connected with inlet andoutlet conduits of the main pump 41. Check valves 22 and 23, openingtoward the main pump 41, are provided in the outlet conduits 20 and 21.The fluid pumped by the feed pump 12 through an inlet conduit 44 out ofan open container 45, is pumped through filter 24 and the one of theoutlet conduits 20, 22 which communicates with the low pressure conduitof the two pressure conduits 13, 14. Assuming that 14 is the highpressure conduit connected with the outlet of the main pump 41, thecheck valve 23 is held closed, and the check valve 22 opens so thatfluid is pumped from the feed pump 12 into the low pressure conduit 13.

The outlet conduit 12a communicates with a control conduit 16 in which aflushing control valve 17 is provided. As schematically shown in FIG. I,and as also shown in FIGS. 2 and 3, the control valve 17 has threepositions, and connects in all three positions, control conduit 16 withthe pressure maintaining valve 18 which has a spring 29 and is operatedby pressure fluid in the shifting conduit 28. In the illustratedposition I], only the control conduit 16 is connected with the pressuremaintaining valve 18, while in the control position I the low pressureconduit 32 is also connected with the pressure maintaining valve 18, andin the position III, the high pressure conduit 30 is also connected withthe pressure maintaining valve 18. Shifting conduits 42 and 43,respectively connected with pressure conduits 30 and I3, and 32, 14,shift the control valve 17 against the action of springs 26 and 27 whichbias control valve 17 toward the normal position II when there is lowpressure in the fluid circulated in the transmission.

Low pressure conduit 13 is connected with a low pressure conduit 30which is connected with control valve 17, and a corresponding highpressure conduit 32 is also connected with control valve 17. A portionof low pressure conduit 30 is connected with a shifting valve 31, and acorresponding portion of high pressure conduit 32 is also connected withthe shifting valve 31, which has two positions I and II. Shiftingconduits 37 and 38 connect low pressure conduit 30 and high pressureconduit 32 with the ends of the shifting valve for operating the samedepending upon which of the conduits 30 and 32 is functioning as highpressure conduit, and which is functioning as low pressure conduit.

A connecting conduit 33 between conduits 30 and 32 has a pressurelimiting, spring-biassed valve 35 operated by fluid pressure in ashifting conduit connected with connecting conduit 34, which alsoconnects conduits 30 and 32 and includes a pressure limitingspringbiassed valve 36 controlled by fluid pressure in a conduitconnected with connecting conduit 33. Pressure limiting valves 35 and 36protect the respective highpressure conduit, for example 32, 14 fromoverloads. When the main pump 41 is reversed, and conduits I3 and 30become the high pressure conduits while conduits 14 and 32 are the lowpressure conduits, pressure limiting valve 36 protects the high pressureconduit 13.

The shifting valve 31 always connects the respective high pressureconduit, for example, conduit 14, 32, with a conduit 39 controlling ahydraulic motor 40 operating the adjusting means of the main pump 41 forvarying the amount of fluid pumped by the main pump 41.

When the pressure in the high pressure conduits 14, 32, 38, 43 issufficiently high, shifting valve 31 is shifted to control position IIand control valve 17 is shifted to position III.

Assuming that conduits 14, 32 are the high pressure conduits connectedwith the outlet of main pump 41, and conduits 13, 30 are the lowpressure conduits connected with the inlet of main pump 41, feed pump 12pumps fluid from container 45 through conduits 12a, 20 and check valve22 into the low pressure conduits 30, 13. Check valve 23 is closed sincethe high pressure from the high pressure conduits 14, 32 acts to closecheck valve 23 against the lower pressure in outlet conduit 41. When apredetermined pressure is reached in the shifting conduit 43, controlvalve 17 is shifted against the action of spring 26 to the controlposition lll.

Due to the fact that a throttle is provided in the control conduit 16between the check valves 20, 23 and the control valve 17, and produces agreater pressure differential than the check valve 22, the fluid pumpedby the feed pump 12 from the open container 45 flows through the checkvalve 20 into the low pressure conduit and into the inlet of the mainpump 41. The same amount of fluid, which was heated during operation ofthe transmission, flows at a point spaced from the main pump 10 throughconduit 30 to the control valve 17 and through the same and the pressuremaintaining valve 18 to the open container 19 where the fluid is cooled.The pressure maintaining valve 18 downstream of the control valve 17determines the pressure in the respective low pressure conduit, assumedto be conduits 13, 30.

When the vehicle rolls, and the hydraulic motor 15 is operated as apump, and the pressure is equalized at the control valve 17, thethrottle 25 will cause an increase of the pressure, which is requiredfor adjusting the main pump 10 at high rotary speeds. This is due to thefact that the pressure at the throttle 25 is transmitted also into thepressure conduit 30. Due to the thereby caused high pressure in pressureconduit 14 and conduit 32, the shifting valve 31 is shifted to positionI, whereupon pressure fluid from conduit 30 flows into the conduit 39and operates the hydraulic adjusting means 40, 10 of the main pump whichis set to an increased amount of pumped fluid.

When the pressure in the hydrostatic transmission varies, for examplewhen pressure conduit 13 is the high pressure conduit, and pressureconduit 14 is the low pressure conduit, the operation of thetransmission takes place in a corresponding manner due to thesymmetrical arrangement of the fluid lines.

FIG. 2 illustrates a control valve 17 which may be used in theembodiment of FIG. 1, and has a longitudinal bore 47 in which a controlslide 46 is slidably mounted. Control slide 46 has piston portions 55,48 at the ends, each of which closes annular ports 49, 90 in the neutralposition of the control slide 46. Chambers 51, 52 are provided at theends of the bore 47, containing springs 53, 54 which hold the controlslide in the neutral position. Pressure conduits 30 and 32 are connectedwith shifting conduits 42, 43, as described with reference to FIG. 1,which supply pressure fluid to chambers 51 and 52 for shifting the slide46 to the two outer control positions I and III out of the normalposition 1. Filter 24 is omitted in the schematic portion of FIG. 2, forthe sake of simplicity.

When in the shifting conduit 43 a greater pressure than in the shiftingconduit 42 prevails, the slide 46 is shifted against the force of spring53 to the left as viewed in FIG. 2, so that pressure fluid can flow outof conduit 30 through the annular port 49, the longitudinal bore 47, andthe pressure maintaining valve 18 into the open container 19. Acorresponding operation of control valve 17 takes place when thepressure in shifting conduit 42 is greater than in shifting conduit 43,due to the fact that main pump 41 is operated in such a manner as tomake conduits 13 and 30 high pressure conduits.

FIG. 3 illustrates the modified control valve 17 in which the slide 56is guided in a longitudinal bore 57 having three annular grooves 58, 59,60. Chambers 61, 62 are provided at the ends of the longitudinal bore57, and contain springs 63, 64 urging the slide 56 to the center normalposition. In contrast to the embodiment of FIG. 2, the pressure conduits30 and 32 directly open into the chambers 61 and 62, respectively, sothat shifting conduits 42,- 43- of the embodiment of FIG. 2 are notrequired. The slide 56 has three piston portions 65, 66, 67.

When in conduit 32 the pressure is higher than in conduit 30, slide 56is shifted against the action of spring 63 to the left, and pressurefluid can flow through annular groove 58 and pressure maintaining valve18 into the open container 19.

From the above description of the drawing it will become apparent thatthe hydrostatic transmission comprises a driven main pump 41, ahydraulic motor 15, high pressure conduit means 14, 32, and low pressureconduit means 13, 30 connecting the main pump 41 with the motor 15; adriven feed pump 12 having outlet conduit means 20, 21 with check valvemeans 22, 23 connecting the feed pump 12 with the low pressure conduitmeans 13, 30, and inlet conduit means 44 connected with a low pressurearea 45; pressure maintaining valve means 18 opening into a low pressurearea 19 for cooling a pressure fluid; a control conduit means 16connecting the outlet conduit means 20, 21, with the pressuremaintaining valve means 18; control valve means 17 in the controlconduit means 16 having a plurality of positions, and connecting saidoutlet conduit means 20, 21, 12a with said pressure maintaining valvemeans 18, and connecting in one of the positions "I also the lowpressure conduit 30, 13 with the pressure maintaining valve means 18 sothat heated pressure fluid from the low pressure conduit means 30, 13 iscooled; and throttle means 25 in the control conduit means 16 betweenthe outlet conduit means 20, 21, 12a and the control valve means 17,producing in the control conduit means 16 a pressure differentialgreater than the pressure differential of the check valve means 22.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofhydrostatic transmissions differing from the types described above.

While the invention has been illustrated and described as embodied in ahydrostatic transmission provided with a throttle, and with the controlvalve connecting in all positions thereof the throttle with a pressuremaintaining valve, it is not intended to be limited to the detailsshown, since various modifications and structural changes may be madewithout departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:

1. Hydrostatic transmission comprising a driven main pump, a hydraulicmotor, high pressure conduit means, and low pressure conduit meansconnecting said main pump with said motor so that said motor is drivenby said main pump; a driven feed pump having outlet conduit means withcheck valve means connecting said feed pump with said low pressureconduit means, and inlet conduit means connected with a low-pressurearea; pressure maintaining valve means opening into a low pressure areafor cooling a pressure fluid; a control conduit means connecting saidoutlet conduit means with said pressure maintaining valve means; controlvalve means in said control conduit means having a plurality ofpositions, and connecting said outlet conduit means with said pressuremaintaining valve means, and connecting in one of said positions alsosaid low pressure conduit means with said pressure maintaining valvemeans so that heated pressure fluid from said low pressure conduit meansis cooled; and throttle means in said control conduit means between saidoutlet conduit means and said control valve means, producing in I saidcontrol conduit means a pressure differential greater than the pressuredifferential of said check valve means.

2. Hydrostatic transmission as claimed in claim 1 wherein said controlvalve means connects in all said positions thereof said outlet conduitmeans with said pressure maintaining valve means.

3. Hydrostatic transmission as claimed in claim I wherein said main pumpincludes means for reversing the flow therethrough so that said highpressure conduit means becomes a low pressure conduit means, and saidlow pressure conduit means becomes a high pressure conduit means;wherein said check valve means connect said outlet conduit means of saidfeed pump with the respective low pressure conduit; and wherein saidcontrol valve means has a normal position connecting only said outletconduit means with said pressure maintaining valve means, and twocontrol positions for connecting also the respective high pressureconduit with said pressure maintaining valve means; and comprisingshifting conduits connected with said low pressure and high pressureconduit means for shifting said control valve means between said normalposition and the control position associated with respective highpressure conduit means.

4. Hydrostatic transmission as claimed in claim 3 wherein said controlvalve means includes a valve slide movable between said normal positionand two outer control positions; springs at the ends of said valve slideurging said valve slide to said normal position; and wherein said valveslide closes one of said shifting conduits before opening the other.

5. Hydrostatic transmission as claimed in claim 3 wherein said controlvalve means connects in said normal position and in said two controlpositions, said outlet conduit means with said pressure maintainingvalve means.

6. Hydrostatic transmission as claimed in claim 1 comprising a filter insaid outlet conduit means of said feed pump upstream of said throttlemeans.

7. Hydrostatic transmission as claimed in claim 1 wherein said outletconduit means of said feed pump include a first outlet conduitconnecting said feed pump with said control conduit, and second andthird outlet conduits connecting a point of said first'outlet conduitupstream of said throttle means with said high pressure conduit meansand said low pressure conduit means, respectively, and wherein saidsecond and third outlet conduits contain check valves, respectively,opening toward said main pump.

8. Hydrostatic transmission as claimed in claim 7 wherein said main pumpincludes hydraulic adjusting means; comprising a shifting valve meansoperated by the pressure fluid in said low pressure and high pressureconduit means to connect said high pressure conduit means with saidhydraulic adjusting means for varying the amount of fluid pumped by saidmain pump.

9. Hydrostatic transmission as claimed in claim 1, wherein said outletconduit means of said feed pump include a first outlet conduitconnecting said feed pump with said control conduit, and second andthird outlet conduits connecting a point of said first outlet conduitupstream of said throttle means with said high pressure conduit meansand said low pressure conduit means, respectively, and wherein saidsecond and third outlet conduits contain check valves, respectively,opening toward said main pump; and comprising a filter in said firstoutlet conduit.

10. Hydrostatic transmission as claimed in claim 9 wherein said controlvalve means connects in all said positions thereof said throttling meanswith said pressure maintaining valve means.

1. Hydrostatic transmission comprising a driven main pump, a hydraulicmotor, high pressure conduit means, and low pressure conduit meansconnecting said main pump with said motOr so that said motor is drivenby said main pump; a driven feed pump having outlet conduit means withcheck valve means connecting said feed pump with said low pressureconduit means, and inlet conduit means connected with a low-pressurearea; pressure maintaining valve means opening into a low pressure areafor cooling a pressure fluid; a control conduit means connecting saidoutlet conduit means with said pressure maintaining valve means; controlvalve means in said control conduit means having a plurality ofpositions, and connecting said outlet conduit means with said pressuremaintaining valve means, and connecting in one of said positions alsosaid low pressure conduit means with said pressure maintaining valvemeans so that heated pressure fluid from said low pressure conduit meansis cooled; and throttle means in said control conduit means between saidoutlet conduit means and said control valve means, producing in saidcontrol conduit means a pressure differential greater than the pressuredifferential of said check valve means.
 2. Hydrostatic transmission asclaimed in claim 1 wherein said control valve means connects in all saidpositions thereof said outlet conduit means with said pressuremaintaining valve means.
 3. Hydrostatic transmission as claimed in claim1 wherein said main pump includes means for reversing the flowtherethrough so that said high pressure conduit means becomes a lowpressure conduit means, and said low pressure conduit means becomes ahigh pressure conduit means; wherein said check valve means connect saidoutlet conduit means of said feed pump with the respective low pressureconduit; and wherein said control valve means has a normal positionconnecting only said outlet conduit means with said pressure maintainingvalve means, and two control positions for connecting also therespective high pressure conduit with said pressure maintaining valvemeans; and comprising shifting conduits connected with said low pressureand high pressure conduit means for shifting said control valve meansbetween said normal position and the control position associated withrespective high pressure conduit means.
 4. Hydrostatic transmission asclaimed in claim 3 wherein said control valve means includes a valveslide movable between said normal position and two outer controlpositions; springs at the ends of said valve slide urging said valveslide to said normal position; and wherein said valve slide closes oneof said shifting conduits before opening the other.
 5. Hydrostatictransmission as claimed in claim 3 wherein said control valve meansconnects in said normal position and in said two control positions, saidoutlet conduit means with said pressure maintaining valve means. 6.Hydrostatic transmission as claimed in claim 1 comprising a filter insaid outlet conduit means of said feed pump upstream of said throttlemeans.
 7. Hydrostatic transmission as claimed in claim 1 wherein saidoutlet conduit means of said feed pump include a first outlet conduitconnecting said feed pump with said control conduit, and second andthird outlet conduits connecting a point of said first outlet conduitupstream of said throttle means with said high pressure conduit meansand said low pressure conduit means, respectively, and wherein saidsecond and third outlet conduits contain check valves, respectively,opening toward said main pump.
 8. Hydrostatic transmission as claimed inclaim 7 wherein said main pump includes hydraulic adjusting means;comprising a shifting valve means operated by the pressure fluid in saidlow pressure and high pressure conduit means to connect said highpressure conduit means with said hydraulic adjusting means for varyingthe amount of fluid pumped by said main pump.
 9. Hydrostatictransmission as claimed in claim 1, wherein said outlet conduit means ofsaid feed pump include a first outlet conduit connecting said feed pumpwith said control conduit, and second and third outlet conduitsconnecting a poinT of said first outlet conduit upstream of saidthrottle means with said high pressure conduit means and said lowpressure conduit means, respectively, and wherein said second and thirdoutlet conduits contain check valves, respectively, opening toward saidmain pump; and comprising a filter in said first outlet conduit. 10.Hydrostatic transmission as claimed in claim 9 wherein said controlvalve means connects in all said positions thereof said throttling meanswith said pressure maintaining valve means.